In recent years, networks such as WPAN (Wireless Personal Area Network) and a sensor network, which include a wireless communication terminal that is compact and consumes low power, are attracting attention. One system that is similar to these is a system which includes an active RF tag that actively transmits a wireless signal.
FIG. 24 is a figure showing one configuration example of a conventional wireless network 700. In FIG. 24, the conventional wireless network 700 includes: a control device 701, and a plurality of terminal devices 702 to 704. The control device 701 is a wireless communication terminal that controls a communication within the wireless network 700. The control device 701 incorporates, in a beacon frame, control information regarding the wireless network 700, and periodically broadcasts the control information. The terminal devices 702 to 704 are wireless communication terminals that communicate with the control device 701 based on the control information. The terminal devices 702 to 704 can use various methods as an access method, and for example, CSMA (Carrier Sense Multiple Access), TDMA (Time Division Multiple Access), FDMA (Frequency Division Multiple Access), and SDMA (Space Division Multiple Access) can be used.
Although the terminal devices 702 to 704 that are used in these wireless networks have a slow transmission speed (from several kbps to several hundred kbps), and a short wireless-signal reaching distance (from several meters to several tens of meters); the terminal devices 702 to 704 are compact and a battery therein has a low-power-consumption capability that can keep driving a terminal device for several years. Improvements have been implemented in a communication protocol and in a frame format in order to reduce a power consumption of the terminal devices 702 to 704; and one example has a configuration that provides an active period for conducting a communication within the wireless network 700, and a non-active period that allows it to enter a sleep mode by not conducting a communication. With regard to the terminal devices 702 to 704, if a prolonged non-active period can be obtained, a prolonged sleep mode can also be obtained; thus the power consumption of the terminal devices 702 to 704 can be reduced.
FIG. 25 shows a superframe period which is one example of a periodical frame configuration. In FIG. 25, the superframe period includes an active period and a non-active period. The active period is a period for conducting communications between the control device 701 and the terminal devices 702 to 704. The non-active period is a period during which a communication is not conducted, and in this period, each of the terminal devices 702 to 704 can reduce power consumptions by entering a sleep mode.
The control device 701 and the terminal devices 702 to 704, all commonly use the active period. The beginning of the active period is used by the control device 701, and the control device 701 broadcasts a beacon frame. The remainder of the active period is used for communications between the control device 701 and the terminal devices 702 to 704, and, for example, CSMA and the like can be used. Furthermore, the active period can be divided into a plurality of time slots, and the slots can be shared between slot CSMA and TDMA. For example, with IEEE 802.15.4 standard, the first half of the time slots are used for a competitive access by CSMA, and each of the time slots in the second half are assigned to be used by a wireless communication terminal to conduct a communication.
The beacon frame includes the control information such as: the number of these time slots and assignment rules of these, a length of the active period, a length of the non-active period, a time until a next beacon frame transmission, and the like.
FIG. 26 shows one example of a communication sequence between the control device 701 and the terminal devices 702 to 704. Referring to FIG. 26, the control device 701 broadcasts a beacon frame 360 at the beginning of an active period 351. The terminal devices 702 to 704 receive the beacon frame 360 and acquire the control information. The control information includes information such as the length of the active period, the length of the non-active period, and the like.
Communications are conducted between the control device 701 and the terminal devices 702 to 704 in the active period 351. For example, the terminal devices 702 to 704 transmit data frames 361, 363, and 365 to the control device 701; and the control device 701 responds with ACK (Acknowledgement) frames 362, 364, and 366.
The communications between the control device 701 and the terminal devices 702 to 704 are not conducted in a non-active period 352. The control device 701 and the terminal devices 702 to 704 can enter a sleep mode during the non-active period 352, and can reduce the power consumption. The control device 701 and the terminal devices 702 to 704 return to an operation mode immediately before the end of the non-active period 352, and prepare for the communication in an active period 353 which is next.
When the non-active period 352 ends, the control device 701 starts the active period 353 which is next, and broadcasts a beacon frame 370. Communications are conducted between the control device 701 and the terminal devices 702 to 704 in the active period 353.
An example shown here is one in which a communication from the terminal device 702 fails. Suppose a case where a data frame 371 is transmitted from the terminal device 702 to the control device 701, but a reception error of the data frame 371 is generated at the control device 701. In this case, the control device 701 does not transmit, to the terminal device 702, an ACK frame in response to the data frame 371.
Although the terminal device 702 will continue waiting for an ACK frame from the control device 701 for a predefined period; when the ACK frame is not returned (i.e. an occurrence of a time-out), the terminal device 702 determines that the transmission has failed. Then, the terminal device 702 attempts to retransmit a data frame 372. Here, suppose a case where the retransmitted data frame 372 has been safely received by the control device 701. In this case, the control device 701 transmits, to the terminal device 702, an ACK frame 373 in response to the retransmitted data frame 372, and the communication ends.
Subsequently, similar operations are repeated, and communications between the control device 701 and the terminal devices 702 to 704 are conducted. A wireless communication system disclosed in patent literature 1 is an example of the system that conducts the communication utilizing the active period and the non-active period described above.